1. Field of the Invention
The invention relates to optical systems and methods, and in particular to systems and methods involving ultra-short, stable, shaped optical pulses.
2. Description of the Prior Art
Various types of optical systems employing optical pulse signals are known in the prior art. High-speed optical computing and communication systems require precisely defined pulses. Future systems will require that these pulses be ultra-short pulses, i.e., pulses on the picosecond to femtosecond time scale with specific temporal and/or spatial intensity profiles for triggering, coding, and analysis.
Several approaches to pulse shaping have been proposed in the prior art that generally use either active or passive shaping techniques; however, these systems have not been applicable to ultra-short pulses, e.g., in the femtosecond pulse width range. Active pulse shaping techniques include electro-optic deflectors and Pockels cells. Passive pulse shaping techniques include mirror stackers, etalon stackers, intensity dependent filters, flat lens shapers, nonlinear interferometers, birefringent filters and double-grating pulse shaping systems.
In particular, an article entitled "Optical Pulse Shaping With a Grating Pair" by J. Agostinelli, G. Harvey, T. Stone and C. Gabel in Applied Optics, Vol. 18, No. 14, 15 July 1979, pp. 2500-2504 discloses the concept of a passive pulse shaping system that uses a pair of diffraction gratings along with various filters of amplitude and/or phase type to alter the temporal and/or instantaneous spectral profile of an input pulse. Amplitude filters will attenuate certain spectral components and therefore certain portions of the temporal profile of the input pulse will also be attenuated. Phase filters will shift various groups of spectral components in time.
As shown in the article, an unshaped input pulse enters the system by impinging upon a first diffraction grating. The diffracted beam emerges as a diverging fan of rays due to the frequency bandwidth of the input pulse and the dispersive nature of the grating. The diverging beam then impinges upon a second diffraction grating of identical groove spacing as the first grating. The angles of the two gratings are precisely matched, so that after the second diffraction, the spectrally separated rays emerge parallel to the input ray direction. A mirror is set perpendicular to the beam emerging from the second grating in order to reflect the beam back through the pair of diffraction gratings, with each ray retracing its steps, so that a collimated beam emerges at the output of the system in the opposite direction of the incident beam.
Each spectral component of the input pulse traverses a different distance in passages through this system. However, due to the negatively dispersive nature of the grating pair, high frequency components of the input pulse emerge prior to the lower frequency components.
In the plane of the mirror, called the filter plane, there is both spatial and temporal transposition of the spectral components of the input pulse. Amplitude filters are inserted in the "filter plane" to attenuate certain spectral components and therefore attenuate certain portions of the temporal profile of the output pulse. Phase filters are inserted to shift various groups of spectral components in time.
Further, the article discloses the use of various opaque strips placed at various places in the "filter plane" to alter the shape of the output pulse and the use of a plate having a continuously varying transmission function to produce a linearly ramped output pulse.
Unfortunately, the output pulse from the system shown in the article is linearly frequency modulated, and as predicted by linear systems theory the Fresnel transform of a band-limited Gaussian is a wider Gaussian with linear frequency modulation. Furthermore, the output pulse is not transform-limited, i.e., the output pulse has more spectral width than is necessary to support the features of the shape of the intensity profile, and it is necessarily longer than the input pulse. Since a transform-limited pulse will propagate a greater distance in an optical fiber than a non-transform-limited pulse before being distorted by dispersion of the group velocity, such a configuration is a substantial drawback in using output pulses from the system disclosed in the article in optical digital communications systems. Also, because of the fact that the shaped output pulse is longer than the input pulse, the full bandwidth of the input pulse cannot be effectively utilized.
M. Haner et al. in an article in Optics Letters 12 (6) June 1987 entitled, "Generation of Programmable, Picosecond-Resolution Shaped Laser Pulses by Fiber Grating Pulse Compression", describe programmable fiber-optic and grating pulse compression techniques. The above article is incorporated herein by reference. The Haner et al. technique uses three steps: (1) the generation of arbitrarily shaped microwave pulses; (2) modulation of a continuous wave laser with a waveguide electro-optic modulator driven by the shaped microwave pulses; and (3) fiber-and-grating pulse compression to shorten the shaped optical pulses to the picosecond regime. By this technique, one must first determine what the nature of the input pulse to the compressor must be to obtain the desired shaped output pulse. Since propagation in the fiber is non-linear, this is a formidable task. In fact, there is no guaranty that an input pulse exists which would result in the desired shaped output pulse, and sufficient control of the desired pulse shaping is difficult. Also, the minimum pulse widths obtainable by this technique are limited. In order to achieve good compression, input pulses should not be much longer than 100 picoseconds. Haner et al. employ 135 picosecond pulses. Since their modulator has a 35 picosecond response, the shaped pulse can only contain a maximum of four features. Furthermore, as pointed out in the article, a maximum compression of only 10-20 is attainable without producing unwanted substructure. This corresponds to a minimum pulse width of 6-12 picoseconds.
Another optical system for pulse shaping is described in our U.S. Pat No. 4,655,547. In that optical system, an input optical pulse is chirped, and the chirped pulse is then passed through an optical component that spatially disperses the frequency components of the chirped pulse and partially compensates the chirp.
The spatially dispersed frequency components are then passed through spatial amplitude and/or phase masks the control and/or adjust the amplitude and/or phase of the frequency components. Finally, the masked components are passed through the first or a second optical component that returns the masked, spatially dispersed frequency components substantially to the spatial distribution of the input pulse while substantially completing the compensation of the chirp to form a transform limited output pulse which can be shorter than the input pulse.
Pulse shapings by the above technique employed prefabricated masks and therefore were not programmable in real time or at high speeds as would be desirable for communications systems. In order to achieve such programmability in that system, a multi-element modulator would have to be associated with the masks and at this time, multi-element modulator technology is immature and quite expensive. At the present time, a multi-element acoustic modulator is available which is limited to only 32 elements at a cost of about $25,000; however, even with the limitation of elements and costs there also exists a problem of crosstalk between elements.
Although such an optical system permits the creation of very short pulses useful in high-speed optical digital communications, there are frequently fluctuations in the width, shape, and energy of pulses formed by such optical systems. Such noise places a limit on the amount of information that may be communicated in such an optical system.
Prior to our copending U.S. patent application Ser. No. 936,488, entitled, "Apparatus for Stabilization of High Speed Optical Pulses", filed on Nov. 26, 1986 and assigned to the assignee hereof, now Pat. No. 4,746,193, May 24, 1988 there has not been a suitable technique for stabilizing very short optical pulses to the extent necessary for some optical communication system applications.
Briefly, and in general terms, that invention provides means for producing highly stabilized pulses from a fiber and grating pulse compressor. An input pulse is propagated in a length of single-mode optical fiber, where the pulse is chirped by the mechanisms of self-phase modulation and group velocity dispersion. In addition, the input power must be sufficiently high to generate significant stimulated Raman scattering (SRS) in the fiber. More particularly, the conversion efficiency to stimulate Raman scattering is preferred to be greater then 10%. SRS clamps the power of the chirped pulse and results in a spectral broadening which is nonsymmetric. The chirped pulsed is compressed in the standard way in a grating pulse compressor. In the compressor, a first grating spatially disperses frequency components of the chirped pulse to produce a spatially dispersed beam. A spectral window (an aperture) is placed into the spatially dispersed beam to act as a bandpass filter. In particular, the window may be asymmetrically disposed to pick an off-center portion of the asymmetric spectrum. Position of the window is varied on a case-by-case basis according to the particulars of the exact asymmetric spectrum. The window passes only those frequency components which are linearly chirped (and hence are suitable for efficient pulse compression) and which are stabilized by SRS. In this way, stabilized, cleanly compressed pulses are produced.
In the copending U.S. patent application of Brackett et al., Ser. No. 065,023, filed June 22, 1987, and assigned to this same assignee, now Pat. No. 4,866,699, Sept. 12, 1989, an optical telecommunications systems is disclosed employing code division multiple access. The systems disclosed therein are incorporated herein by reference and are suitable for use with the ultra-short shaped pulses of the present invention.